1. Field of the Invention
The present invention generally relates to the field of high voltage cables manufacturing. More specifically, the present invention relates to a valve device particularly adapted to be used in feeding apparatuses for dispensing products, such as for example polymer compound granules, to an extruder machinery carrying out high voltage cables insulation.
2. Overview of the Related Art
Cable manufacturing, and in particular the manufacturing of high voltage cables—such as, for example, high voltage electric power distribution cables operating at voltages in the range from 20 up to 500 kV—typically requires that a central conductor of the cable is completely covered by a coating of homogenous insulating material, referred to as cable insulator in the following, able to electrically insulate the central conductor and prevent any possible dangerous electric discharge towards the surrounding environment.
As known in the art, during the manufacturing of a high voltage cable, the step of covering the central conductor with the cable insulator is achieved by an extrusion process of polymer compounds substantially including raw thermoplastic materials in the form of pellets (generally known as “granules”), which extrusion process allows creating a coating of a substantially constant and predetermined cross-sectional profile through a machinery called extruder. In particular, the granules of thermoplastic material, which are to be fed to the extruder, are loaded, stored and transported within a bin (generally an octabin,—i.e., a prism-shaped bin having an octagon base); in order to feed the extruder, the octabin is placed over an extruder load hopper, which connects the octabin and the extruder and through which the granules of thermoplastic material are caused to flow by gravity. When the granules enter the extruder and come into frictional contact with an extruder rotating screw (normally rotating at up to 120 rpm), the granules of thermoplastic material are heated up (for example at temperatures from 200° C. up to 275° C.) and melt, and the melted polymer is then pushed through an extruder die having a desired cross-section, and finally properly cooled.
As known to those skilled in the art, it is of the utmost importance that the cable insulator is, as far as possible, free from contaminants, so as to be able to ensure an effective insulation of the high voltage cable. In fact, the presence of contaminants in the cable insulator causes defects which may cause, in relative short periods of use, a drastic increase of local electrical fields acting thereon in response to applied high voltage gradients; such condition typically accelerates the ageing of the cable insulator and reduces an insulation breakdown strength of the same, thus making the high voltage cables unreliable. To this end, the raw material used to form the cable insulator, i.e. the granules of thermoplastic materials fed to the extruder, shall have a very high degree of “cleanliness”. When the octabin is located above the extruder load hopper, the risk that contaminant particles or foreign bodies spuriously enter into the load hopper is relatively low, since the octabin substantially covers the extruder load hopper, whereas such risk becomes significant when the octabin is positioned away from the extruder load hopper and the latter is left uncovered; this actually happens relatively frequently, when, for example, the octabin has to be removed because for example empty, and replaced by a new octabin.
Therefore, although the above described method used for feeding or discharging the granules of thermoplastic material from the octabin to the extruder during cable insulator manufacturing is widely practiced thanks to the simplicity thereof, the fact that it is not possible to effectively prevent contaminating particles coming from the surrounding environment from entering the extruder when the octabin is not above the extruder load hopper makes the cable insulator (and hence the whole cable) very instable in terms of electrical and mechanical performance, thus resulting in a reduced reliability and lifetime of the cable.
Some manual expedients could in principle be adopted to prevent the input of contaminating particles into the extruder load hopper; for example, it could be possible to provide a cap for the extruder load hopper which is manually put thereon to close its entrance when the octabin is not in place, and manually removed after the octabin has been positioned so as to allow the granules of thermoplastic material to fall into the load hopper. However, such a solution is not very effective, since it involves a further manual operation to be performed by the personnel during the cable manufacturing process, thus being burdensome for an operator who has to be extremely careful during accomplishing the putting and the removing operations. Moreover, the manual cap placement and removal is an operation prone to errors, being inevitably subject to human mistakes that may impair the overall cable insulator manufacturing process, thereby causing time wastes and/or economic losses. Last but not least, the manual cap placement and removal may cause injuries to the operator.
For example, WO2009076375A1 discloses a dispensing system for granular materials, in particular agricultural products, capable of dispensing multiple products in different receptacles through a rotating structure. The rotating structure disclosed in such documents, although acting as a rotating valve for selectively distributing granular materials into corresponding receptacles, does not avoid possible feeding of undesired contaminating particles or materials in the receptacles. In particular, if undesired materials reach the dispenser, the latter keeps on supplying the contaminated granular materials without any supplying stop mechanism. Moreover, the dispenser configuration allows switching the material in different receptacles, but it does not allow discharging waste products outwards; in fact, since no discharge means are provided, which could throw away the contaminated granular material, such dispenser configuration is not adapted to be used for the purpose of the present invention, i.e., for feeding the extruder load hopper (and hence the extruder) during high voltage cable manufacturing.
US2003/159658 relates to a product discharge mechanism for a hopper having a gate or valve slideably positioned such that in one position it covers a fixed discharge opening of the hopper thereby obstructing product flow, and in one or more other positions it is at least partially opened with respect to the discharge opening so that said product may flow through the fixed opening and thereby be discharged from the hopper. In other words, with the mechanism disclosed in such document, no product can flow when the valve is in the closed position, but when the plate is away from the fixed openings, product can flow.
Therefore, the valve disclosed in such document, although regulating the product flow, is not able to prevent any product contamination by contaminating particles inevitably coming from the external environmental, i.e., the drawback of feeding waste materials into the hopper persists. Moreover, no lateral discharge passageways are provided for the product discharge mechanism so as to switch the undesired material to the external.